The present embodiments relate to a differential voltage measuring system, a differential voltage measuring system for measuring a respiration movement, a differential voltage measuring system including a driver circuit for the right leg, and a method for differentially measuring voltages.
During the measurement of bioelectric signals (e.g., of ECG signals), common-mode interference signals (e.g., interference as a result of common-mode signals) occur as a result of non-ideal measurement inputs of an ECG measuring arrangement. The signals arise, for example, from the power supply frequency at 50 Hz. Common-mode interference signals occur if non-identical conditions such as different impedances and capacitances occur at the two measurement inputs during the differential ECG signal measurement. One example of a conventional measuring arrangement for measuring an electrocardiogram is shown in FIG. 1. In fact, common-mode signals (e.g., interference signals) are not concomitantly amplified during the differential measurement, and so the common-mode signals are suppressed. However, the different impedances of the inputs of the ECG measuring arrangement have the effect that different input signals caused by the same interference signal are present at the two inputs of an amplifier circuit of an ECG measuring arrangement, such that the interference signal is amplified together with the actual measurement signal. These common-mode interference signals are very strong in the application on a patient (e.g., a human being or an animal), since the electrode contacts on the skin of the patient have a greatly varying quality factor without complex preparation. An electrode contact on the patient may have impedances of between 10 kohms and several megohms and likewise greatly varying capacitances. As a result, the difference between the impedances and capacitances at two measurement inputs is also in the range of up to several megohms. One example of an ECG signal subjected to common-mode interference by an impedance difference of 500 kohms is shown in FIG. 2. In some instances, the impedance differences at the inputs of the ECG measuring arrangement are even higher, such that an evaluation of the ECG signal hardly appears to be possible any longer.
By contrast, the total sum of the impedances of the electrode contacts is hardly of importance any longer on account of progress in the art with input impedances of from hundreds of megohms to several gigaohms; it is completely irrelevant to the common-mode interference signals.
Conventionally, there are various procedures for suppressing common-mode noise.
One possible procedure includes the measurement of a common-mode current and an adaptive filtering adapted to the measured common-mode current. In this case, the common-mode current is measured separately with the aid of two separate electrodes. The two electrodes are arranged at a distance from the heart and in spatial proximity to one another, such that only common-mode signals are measured by the two electrodes. However, the common-mode measurement electrodes and the ECG measurement electrodes have different impedances, such that the measured common-mode signal may not simply be subtracted from the differential signal measured at the ECG measurement electrodes. Therefore, with the aid of an adaptive filter, the different transfer function of the electrodes is simulated, and the common-mode interference signal is filtered out of the differential measurement signal present at the ECG measurement electrodes. However, this procedure functions only during the suppression of non-correlated common-mode signals. If correlated common-mode signals occur, filtering the common-mode signals also leads to filtering out or attenuation of the measurement signals or useful signals.